1. Field of the Invention
The present invention relates to printer systems, and more particularly, to an improved printer system having an optical sensor device for detecting positioning and alignment of print media and determining correct printing position and alignment thereon.
2. Art Background
Many ink jet printers print while moving in both directions, that is, left to right and then right to left across the paper or other print media. It is thus important for the printer to be able to line up text and graphics on a page so that it is printed in the desired position on the paper independent of the direction of movement of the print head. Presently, when an ink jet print head is calibrated to make such proper alignment, a trained technician monitors the movement of the print head backward and forward across the print media, and then manually adjusts the print head moving means to compensate for any inaccuracies. Several inaccuracies are taken into account in this process, including system backlash from the gearing in the stepper motor system which drives the print head, the stretching of the belt of the print head drive system, and mechanical stress of the support members which support the belts and pulleys of the drive system.
Generally prior to shipment, or immediately upon installation, a technician calibrates the ink jet printer by detecting what the printer believes is the center of a line in both forward and backward movement of the print head, and then by manipulation of a plurality of dip switches which control the movement of the print head and adjust the movement of the print head so that the center position of the print head is properly aligned for both forward and backward movement thereof.
It has also been found that, particularly with respect to ink jet printers, that as the printer wears, there is increased flexure of the support members which support the print head and increased stretching of the belt which moves the print head. As a result, the printer head believes it is in a particular position, but in fact, it is displaced by the amount of flexure in the support members. The effect of this error is shown diagrammatically in FIGS. 2a and 2b. In FIG. 2a it is shown that the support members and belts are relatively rigid, and therefore, after initial calibration, the alignment of the print in both directions is correct. In FIG. 2b, it is shown that the increased stretch and flexure of the system causes the print to misalign.
Thus, there are several drawbacks to this type of system. First, the dip switches are required, which adds cost and complexity to the manufacture of the printer. Second, the manual calibration of the printer requires time and the input of a skilled technician, the accuracy of which is dependent upon the skill of the technician. Third, as the printer ages over time, the stretch of the drive belt and the flexure of the mechanical support members changes, and in particular, the stretch and flexure increases. As a result, the alignment of the print from the print head travelling in opposite directions changes, causing misalignment and less than desirable image quality. Consequently it is necessary to recalibrate the printer over time as the calibration drifts.
There are two general types of drive head movement control systems which are presently employed. The first is a timing based control system which measures the time a print head is moving in a particular direction, and then, knowing the speed of the print head and the beginning point of the movement from either side of the page, the system determines the location of the print head. The second type of print head movement control system relies upon a magnetically encoded or optical position detection means which indicates the position of the print head relative to the print carriage.
Prior art serial type printers presently have incorporated therein a photoelectric sensor disposed on the print head to detect the home position of the carriage. However, such photoelectric sensors are not currently used in this type of printer to detect any information relative to the positioning and alignment of paper or other print media disposed in the printer. Thus the present invention seeks to overcome this and other disadvantages of the prior art serial printer systems.